Vacuum tube electrode



Nov. 28, 11950 M. ARDIT! 2,531,382

VACUUM TUBE ELECTRODE Filed June 5, 1947 COEFFICIENT OF SECONDARY EMISSION a 100 200 500 400 5'00 was 1 N VEN TOR. MALI/W65 A/RO/ 7'/ A TTOR/VI'Y Patented Nov. 28, 1950 VACUUM TUBE ELECTRODE Maurice Arditi, New York. N., Y., assignor to International Standard Electricilorporation, New York, N. Y., a corporation of Delaware Application June 3, 1947, Serial No. 752,207 In. France August 29, 1939 Section 1, Public Law 690,, August 8, 1946 Patent expires August 29, 1959 6 Claims.

.activated, i. e., their secondary emission power is increased, by a thermal treatment which causes a substantial change in the appearance of the electrode surfaceand, by virtue of a preferential migration phenomenon, is believed to change the distribution therein of the component elements,

particularly the distribution of copper or an equivalent heavy element when it is present in the alloy. Although the theory of operation of this phenomenon is not now wholly clear, practical. experience indicates that a relationship may exist between this phenomenon and the electrode activation which is reflected in much greater secondary emission exhibited by such electrodes compared with that of electrodes made of an alloy not so treated, or made according to prior art with surface layers containing compounds of caesium, silver or the like.

In accordance with this invention, electrodes havine these desired properties of high secondary emission are prepared by processing alloys which contain one or more light metals of the group consisting of aluminum, magn sium and bervllium, and which preferably but not necessarily also contain a metal of the copper family. For example. an allov which is especially suitable for use in the practice of the present invention is a Duralumin allo having the a proximate composition: aluminum 95%, copper 4%. manganese 0.5 magnesium 0.5%, and traces of iron. However. ithas been found that the principles of the present: invention can be utilized in treatment of alloys irrespective of their composition provided they include appreciable proportions of the elements above mentioned.

In connection with this invention, it was found that electrodes made from alloys of the ty e above mentioned, when examined after cleaning and polishing, and prior to thermal treatment, exhibit-a coeflic'ient of secondary emission only very 55 trodes at progressively higher temperatures in treated electrodes does not change appreciably when the speed of impact of the primary electrons is changed provided that it is at least. 100 volts. It was further found that if an electrode made of one of these alloys is heated at a relatively low temperature in an oxidizing atmosphere (pure oxygen or air), the coefiicient of secondary emission initially decreases, although the slope of the curve representing variations of secondary emission as a function of the primary electron speed remains substantially the same as that of an untreated electrode.

However, it is now found according to the present invention that if the temperature at which the electrode is heated in the oxidizing medium is materially increased, the appearance of the electrode surface changes, become dull, and the coefficient of secondary emission of the electrode begins to increase without appreciable change in the slope of the curve representing secondary emission variation as a function of the impact speed of the primary electrons. As the temperature of treatment is further increased, the electrode surface appears to become granular, and the metal begins to swell or pun. .At this point, coeflicient of secondary emission increases ver rapidly, reaching, for example, a Value of 10 for an impact speed of 400 volts: and at the same time the curve of secondary emission variation as a f nction of primary speed changes its slope, passing the maximum, which is generally very peaked. for speeds in the neighborhood of 500 to 550 volts.

A characteristic of these electrodes thus obtained is that the coefficient of secondary emission remains the same even if, after preparation and mounting in a vacuum tube, the electrodes are operated at a temperature which may reach 400 C.

In accordance with this invention, thermal treatment of the electrodes to cause activation as described is performed before they are mounted inthe tubes in which they are to be used and preferably consists of uniformly heating the elec- 3 pure oxygen or in air. Heating may be caused by electric resistance or high frequency induction, or if desired by employing a continuous or discontinuous discharge between the electrodes to be treated and auxiliary electrodes, or it may be efiected by use of a heated stove through which a current of oxygen or pure air is circulated.

The end point of the activation treatment is determined by examining the electrodes periodically to note the changes in appearance mentioned above. It is particularly to be noticed that the swelling of the electrode surface initially proceeds very rapidly, and the time to stop heating is when the electrode surface becomes very considerably swollen, as also occurs when too high a temperature is applied, and after such treatment the electrodes merely exhibit a coeflicient of secondary emission of approximately the same order as that of an untreated electrode.

After the heat treatment to cause activation as above described, the electrodes may be installed in a discharge tube and degassed in the usual manner by heating under vacuum to 350-400 C. for about three-quarters of an hour. This degassing operation, as is well known, is desirable if a hard 01' high vacuum is to be necessary in the tube wherein the electrodes are to be used. It 'does not afiect the secondary emission coeflicient of the processed electrodes.

The accompanying drawing illustrates typical curves representing the relationship of secondary emission coefficient to primary electron speeds. The curves numbered i and 2 indicate the variation of secondary emission coefficient as a function of primary electron speed when the tested "electrode was made of a Duralumin alloy which was not subjected to the activation treatment according to this invention. The other curves numbered 3 through l, represent corresponding characteristics of electrodes that have been sub- -of activation as represented by the curves numbered 6 and I.

Although the electrodes prepared according to this invention exhibit unusually large secondary emission coefficients, which may be as high as 10-15, it is of interest that no inertia has been, observed in the secondary emission phenomenon; 'i. e., when the primary electron stream is interrupted, secondary emission immediately ceases, which indicates that here the secondary emission is a normal, not an abnormal phenomenon, in con-.

trast-to the experimental studies described by Malter in The Physical Review, 5, 48 et seq.

It is to be understood that in this specification and in the accompanying claims where the appearance of a processed electrode is described as being slightly granular, it is meant that the electrode has been processed under conditions such that a polished surface thereof during the processing becomes dull and lacking in luster and that after further processing the surface becomes 'matte in appearance due to the formation of minute pits or bumps upon the surface.

It is this last condition that constitutes the end-point of the activation process herein described.

electrode To facilitate an understanding of the present invention, a specific example of how the invention may be practiced will now be described, but it clearly to be understood that this example is provided by way of illustration, not by way of limitation of the invention.

Example An alloy is prepared having the following composition based on weight:

Percent Aluminum 95 Copper 4 Manganese L 0.5 lvfagnesium 0.5

A piece of this alloy was fabricated into an electrode by the usual rolling, stamping, cleaning and polishing operations and the electrode thus obtained was mounted on a support and placed in a chamber containing oxygen for heat treatment. The electrode was then heated at progressively elevated temperatures until the surof the electrode appeared slightly granular, at which time the treatment was immediately discontinued and the electrode was removed from the chamber. When tested for. secondary emission power, this electrode was found to exhibit an unusually high ratio of electrons emitted from compared with the number of electrons impinging thereupon.

Having thus described the present invention, what it is desired to secure by Letters Patent is:

7.. In a process for the manufacture of a vacuum tube electrode having an abnormally high second ary eml sion coefficient, the improvements that comprise selecting an alloy containing a predominant proportion of a metal selected from the class consisting of aluminum, magnesium and beryllium; forming an electrode from said selected alloy; and heating said electrode in the presence of oxygen and at a temperature such that a polished surface of the alloy, would become slightly granular in appearance.

' 2. In a process for the manufacture of a vacuum tube electrode having an abnormally high secondary emission characteristic, the improvements that comprise selecting a predominantly aluminum alloy, forming an electrode from said selected alloy, and heating said electrode in the presence of oxygen and at a temperature such that a polished surface of the alloy would become slightly granular in appearance.

3. In a process for the manufacture of a vacuum tube electrode having an abnormally high secondary emission characteristic, the improvements that comprise selecting a predominantly beryllium alloy, forming an electrode from said selected alloy, and heating said electrode in the presence of oxygen and at a temperature such that a polished surface of the alloy would become slightly granular in appearance.

4. In a process for the manufacture of a vacuum tube electrode having an abnormally high secondary emission characteristic, the improvements that comprise selecting a predominantly magnesium alloy, forming an electrode from said selected alloy, and heating said electrode in the presence of oxygen and at a temperature such I that a polished surface of the alloy would become slightly granular in appearance.

5. A vacuum tube electrode having an abnormally high secondary emission characteristic,

REFERENCES CITED The following references are of record in the file of this patent:

Number Number 8 UNITED STATES PATENTS Name Date Kelly Oct. 24, 1922 Rentscher May 27, 1930 Warnecke Feb. 13, 1940 Nelson Jan. 29, 1946 FOREIGN PATENTS Country Date Great Britain July 29, 1941 Great Britain Feb. 25, 1944 

